For three decades the electron microscope has been an important part of the research program of the Epidemiology Section of LID focussed on the study of enteric diseases. Highlights of these investigations have included inquiry into the etiology of diseases of unknown cause and studies with fastidious agents of disease that defied cultivation in any cell culture system. The electron microscope, as a unique instrument played a major role in the discoveries of the 27nm Norwalk virus and the establishment of its etiologic role in human diarrheal disease and of the 27nm hepatitis A virus particle and establishment of its etiologic role. Although intensive efforts have been made by numerous groups of investigators, the Norwalk and related human enteric caliciviruses have yet to be cultivated in any tissue culture system. This, along with the recent expression of 27nm virus-like particles of some members of this group of agents, as well as studies of expressed viral proteins of rotavirus, have contributed to the continuing important role of the electron microscope in the research program of the Epidemiology Section. The electron microscope continues to be the only method (i) for direct detection of the Norwalk and related viruses and derivative 27nm expressed virus-like particles and (ii) for unraveling the antigenic relationships among these fastidious agents. The electron microscope was used in individual experiments as an adjunct to other studies such as determining the morphology of recombinant baculovirus expressed rotavirus virus-like particles, examining for self-assembly of baculovirus- expressed capsid proteins of astrovirus and mink enteric calicivirus, examining suspensions for the presence of viruses from individuals who developed signs and/or symptoms following administration of candidate vaccine Wa x (DS -1 x UK), examining stool suspensions for virus particles from an individual who developed severe diarrheal illness, examining NSP2 preparations, colloidal gold studies with feline calicivirus.